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K9ZMD
04-23-2008, 03:57 AM
Wanted to make a temporary 6 meter dipole to use at my dad's place in Wisconsin, but can't make any holes for transmission line. Dad's wooden, awning-type windows will close OK on flat line, making a folded dipole the natural choice. So I tore into an unopened 40 foot pack of 300 ohm TV twin lead that has been gathering dust in the junk box since 1973. Nice thin stuff, it will fit beneath the window just fine.

Once built, I suspended the dipole horizontally in the clear at about 14 feet. The 30 foot feedline dropped straight down for over 1/4 wavelength, then ran horizontally away from the antenna (maintaining the 90 degree angle) about 3 feet above the ground.

I fastened the feed line to a 4:1 balun and connected that to my antenna analyzer through about 2 feet of RG-58. The formula dimension resonated at 49.750, so I whacked 7/16" off each end and tested it again. This time, perfecto. Around 50.125 - there is no at with my antenna analyzer - I saw the following: R=50-51, X=4-7, Z=50-52, and VSWR of 1.1:1. Somewhere north of 52 MHz the VSWR rose to 2:1.

Naturally I'm pleased with all of those figures, since I actually expected Z to be more like 75 ohms. The 50 ohm impedance may be due to antenna height, or a fortunate length of transmission line (or both). I won't look a gift horse in the mouth. :)

Now, for my question - and for this we have to go back to my earlier, casual mention of "nice thin stuff". This twin lead is made of #24 stranded and the ribbon is only about 1/4" wide. How much power can I risk? We've had some discussion threads about how much power TV twin lead can carry, but I don't really think folks were talking about this sort of Rat Shack indoor stuff.

I don't mind running QRP during 6 meter openings, that's actually what I figured to do when I built the antenna. Then, after seeing such promising numbers showing up on the antenna analyzer, I can't help but wonder if I could just run my IC-706 MIIG as if I had a real antenna connected to it. What do you think? Anyone out there had experiences, good or bad, with light weight twin lead?

Gary, K9ZMD

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-------- AND HERE IS WHAT I LEARNED ---------------------------

OK, my final post to this thread - I am also editing the lead post to add this info, for your convenience.

Once installed at about 20 feet at Dad's house, this light weight folded dipole had a VSWR of about 1.6:1 as measured by SWR meter in my IC-706MIIG. That is close to what I would expect when using 4:1 balun between RG-58 and the balanced line feeding a (approx) 300 ohm folded dipole.

After initial testing at low power, I risked running 100 watts, key down for about 30 seconds. No fireworks and no apparent heating of the balun or the 300 ohm ribbon line. So, that pretty well answers the question I originally asked about using such light weight twin lead for a transmitting antenna.

Thank you all once again for your suggestions and comments. 73

Gary, K9ZMD
Temporarily at Neenah, WI

W4INF
04-23-2008, 04:07 AM
Im guessing 50w CW, 100w SSB and 25w PSK, because of there mode demands on RF output duration.

Anyone else have ideas?

Andrew

K7JEM
04-23-2008, 05:03 AM
Key up with about a 50 watt FM carrier for about 30 seconds, then unkey and see how hot the antenna has become. If it burns your hand, back off the power. If it's warm, it should be good to go for 100W SSB.

Joe

KC7YPJ
04-23-2008, 05:55 AM
I've had 1 of my hb vertical antenna fed with basicaly the same cheesy twin lead (ultra thin 300ohm twinlead from ace) suspended about 5-8' off the ground to a tuna in the shack for a year now, full duty cycle on ssb and cw, and 75-80 watts on psk31/rtty/olivia and other digital modes for an hour or 3 with no feedline heating at all.

given the multiband (resonance? we don't need no stinking resonance) use of the 39' conduit monopole with 32 each 50'/25'/12.5' radials and the means of feeding it I would say this is probably one of the more extreme uses you can find for balanced line regardless of quality, the cheapy ace stuff is still working fine, even after a less than friendly Idaho winter.
You shouldn't have any issue's with treating it like a "real" antenna, after all thats what it is. if your not convinced do the fm carrier test mentioned and see for yourself, I can't imagine this "crap" I have beeing any worse, as I have yet to find cheesier line than this stuff.

K9ZMD
04-23-2008, 06:51 AM
OK gang, thanks for the encouragement. I've got my stuff packed & ready to head for Wisconsin tomorrow. Once I get there, I'll string this little folded dipole up and do some cautious testing, as suggested, to see what occurs. Whatever happens, I'll update this thread with a report.

I'll also be active from Wisconsin on 40-10 meters CW & digimodes, using a doublet that has somewhat heftier twin lead for a transmission line.

Thanks again for posting your suggestions. 73

AD5ND
04-23-2008, 07:21 AM
Just a thought. Try cutting a piece of 2x4 the with of the window and drilling holes in it for your feed line, then close the window on the board.

N1BHH
04-23-2008, 04:41 PM
Here is a great window feedthrough. It handles open wire or twinlead feeders, coax and ground connections.

http://www.universal-radio.com/CATALOG/antsup/4748.html

KA4DPO
04-23-2008, 06:45 PM
Gary, a well matched antenna and feed line made from light duty TV twinlead can handle 300 or 400 watts easily. The only concern is where it comes through the window. If there is metal flashing there may be some arcing and sparking at high power levels. If your only running 100 watts then don't even give it a second thought, just run it.

John..

PS, you have my permission to come here and kick my butt if I'm wrong.

G0GQK
04-23-2008, 09:27 PM
I thought I'd seen everything on QRZ, but never did I ever see tuner spelt as tuna !

G0GQK

K4AX
04-23-2008, 09:43 PM
Exactly dead on.


Gary, a well matched antenna and feed line made from light duty TV twinlead can handle 300 or 400 watts easily. The only concern is where it comes through the window. If there is metal flashing there may be some arcing and sparking at high power levels. If your only running 100 watts then don't even give it a second thought, just run it.

John..

PS, you have my permission to come here and kick my butt if I'm wrong.

K7KBN
04-23-2008, 09:46 PM
I thought I'd seen everything on QRZ, but never did I ever see tuner spelt as tuna !

G0GQK

Noticed something fishy there, did you? :D

K0RGR
04-23-2008, 11:30 PM
I wonder what part of Wisconsin you're headed for? There's an active group in the Eau Claire/Chippewa Falls area on 50.135 . A CQ on that frequency will often draw a response.

K8JD
04-23-2008, 11:57 PM
Youvshould have no problem with that nicely matched antenna and line for 6M. Just don't try to use it on 160 thru ten M also ! ! !

K9ZMD
04-24-2008, 04:11 AM
Thanks for the comments added since last night. I did manage to get to Wisconsin today. That's not always a certain thing because I travel company space-A, thanks to my son, the flying bus driver. Let me tell you sometime about once spending 30 hours at SFO to watch 11 planes leave for ORD without me, before I gave up and hopped back to Palmdale.

OK, no sweat about not trying to multi-band this folded dipole; I know this is generally a band-specific design. Actually, I already have a doublet here for 40-10, using some heftier balanced transmission line.

The suggestions to make a window insert with bulkhead connectors, etc, is a good one for sliding & sash-type windows, but Dad's windows are the awning-type, which are hinged at the top and swing outward from the bottom. Not only does that create a gap at the bottom, it also creates a long wedge-shaped opening at each side. Plugging that opening would best be done with a full frame insert, and that's a bigger project than I want to tackle for just a few short-term set ups each year.

And - truthfully - I think I'm rejecting anything that would indicate my stays in Neenah are becoming full-time care for Dad, with only short visits to my own family in Palmdale. :(

I suspect that the Eau Claire, Chippewa Falls area would be a long stretch from Neenah with just a dipole . . . maybe even with a Yagi under most conditions. Only time and a couple of sporadic E openings will tell what its normal reach will be.

I'll be back to report on how well that folded dipole works (or doesn't work) when I run the IC-706 MIIG at full power on SSB. Yeah, based on comments here, I'll at least risk giving it a try. Thanks again & 73

Gary, K9ZMD

K9ZMD
04-30-2008, 01:04 PM
OK, my final post to this thread - I am also editing the lead post to add this info, for your convenience.

Once installed at about 20 feet at Dad's house, this light weight folded dipole had a VSWR of about 1.6:1 as measured by SWR meter in my IC-706MIIG. That is close to what I would expect when using 4:1 balun between RG-58 and the balanced line feeding a (approx) 300 ohm folded dipole.

After initial testing at low power, I risked running 100 watts, key down for about 30 seconds. No fireworks and no apparent heating of the balun or the 300 ohm ribbon line. So, that pretty well answers the question I originally asked about using such light weight twin lead for a transmitting antenna.

Thank you all once again for your suggestions and comments. 73

Gary, K9ZMD
Temporarily at Neenah, WI

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